Tissue sealants are increasingly important adjuncts in surgical procedures, being used in fields such as vascular surgery, cardiac surgery, spine surgery and brain surgery as well as in general surgery. Uses for tissue sealants include, among others, augmenting or replacing sutures to join tissues or place them in proximity, closing perforations in biological membranes to prevent leakage of fluids, incorporating medicinal substances at the location of emplacement for localized release, and filling areas of tissue removal. One commonly used tissue sealant is fibrin glue, a material analogous to clotted blood, which is obtained from reaction of fibrinogen and thrombin isolated from blood plasma. For example, see “Fibrin Glue from Stored Human Plasma: An Inexpensive and Efficient Method for Local Blood Bank Preparation,” William D. Spotnitz, M.D., Paul D. Mintz, M.D., Nancy Avery, M. T., Thomas C. Bithell, M.D., Sanjiv Kaul, M.D., Stanton P. Nolan, M.D. (1987), The American Surgeon, 53, 460-62. However, concern about possible viral or prion contamination of human blood-derived protein products, and dissatisfaction with the relatively long times often required for fibrin gelation or “setting” to occur, have resulted in a search for tissue sealants with more advantageous properties.
There have been systems developed that use fibrin glues as part of a more complex assembly with more favorable properties. U.S. Pat. No. 6,699,484 discusses the use of fibrinogen in mixtures with polysaccharides such as hyaluronan and chitosan to form surgical adhesives, wherein the fibrinogen and thrombin components are distributed in dry form on a support comprising the polysaccharide, which is activated by water when emplaced on a wound to form a sealant.
In an attempt to avoid the use of human blood products, other mammalian sources of proteins have been studied. A tissue sealant has been prepared using bovine serum albumin that is crosslinked with glutaraldehyde. An example is BioGlue Surgical Adhesive® produced by CryoLife, Inc. of Kennesaw, Ga. However, bovine tissues are also a source of concern in terms of the possible presence of pathogenic entities such as viruses or prions. The types of processing required to destroy viruses or prions also tend to denature the desired proteins and make them intractable as sealants.
A tissue sealant that does not use proteins isolated from mammalian blood, such as Duraseal® produced by Confluent Surgical Inc. of Waltham, Mass., comprises tri-lysine-amine and an activated polyethyleneglycol. A similar product, termed CoSeal® and produced by Baxter of Deerfield, Ill., is likewise composed of synthetic functionalized polyethyleneglycol derivatives, also avoiding the use of blood-derived materials. However, both of these synthetic hydrogels are dimensionally unstable in the presence of water, undergoing considerable swelling. For example, see “Evaluation of Absorbable Surgical Sealants: In vitro Testing,” Patrick K. Campbell, PhD, Steven L. Bennett, PhD, Art Driscoll, and Amar S. Sawhney, PhD, at www.duralsealant.com/duralsealant/literature.htm (as of Aug. 24, 2006). This tendency to swell can be highly disadvantageous in certain applications, such as neurosurgery, where the resulting pressure on nerve or brain tissue can produce serious side-effects.
Chitin, a biopolymer that is abundant in the shells of arthropods, is a β-1,4 polymer of 2-acetamido-2-deoxyglucose. During its isolation, it is freed from proteinaceous and mineral components of the shell. Purified chitin can be further processed by chemical treatment resulting in deacetylation to yield chitosan, (poly-(2-amino-2-deoxyglucose)), which is a basic (alkaline) substance due to its free amino groups. From the perspective of medical uses, chitosan offers several desirable properties. The material is known to be non-toxic and biocompatible, and since chitin is not derived from vertebrates and is processed under rather harsh conditions such as exposure to alkali during its transformation into chitosan, the possibility of contamination with viruses or prions that are pathogenic to mammals is very low. The utility of biocompatible chitosan derivatives in medical applications has received attention. For example, U.S. Pat. No. 5,093,319 discusses the use of films prepared from carboxymethylated chitosan for use in surgery to prevent post-operative adhesion of injured soft tissues upon healing. The chitosan derivatives are described to be formed into a biodegradable “sheet” that during surgery is emplaced between soft tissues for which adherence during healing is not desired. In another type of use, U.S. Pat. No. 4,532,134 discusses the use of chitosan in promoting blood coagulation in wounds.
Hydrogels are gels in which water is the dispersion medium. A common example of a hydrogel is a gel formed from the protein gelatin in water. Other hydrogels are formed by polysaccharides such as agar dispersed in water. Hydrogels in the form of sheets are used as wound dressings, where they are favored for their ability to help maintain a moist environment to facilitate healing of the wound without drying and cracking of tissues. For example, see www.medicaledu.com/hydrogellsheet.htm. Chemical derivatives of chitosan have also been used to form hydrogels for use as surgical sealants and in drug delivery devices. U.S. Pat. No. 6,602,952, assigned to Shearwater Corp., describes the preparation of poly(alkyleneoxide)chitosan derivatives and their use in the formation of hydrogels. The addition of these hydrophilic but non-ionic groups to the chitosan molecule alters its physical properties. Poly(alkyleneoxides) such as poly(ethyleneoxide), also known (somewhat inaccurately) as poly-ethyleneglycols or PEGs, are formed by the polymerization of alkylene oxides (epoxides) such as ethylene oxide. They may be obtained in a wide variety of molecular weights, with various structural features such as activated end groups, hydrolysable linkages, and others. For example, see the Nektar PEG catalog that lists a wide variety of the Shearwater functionalized PEGs, at www.nektar.com/pdf/nektar_catalog.pdf (as of Aug. 24, 2006).
Other methods have been described for the preparation of hydrogels from chitosan. The published PCT application WO2005/113608 and the published U.S. patent application no. 2005/0271729, both by the same inventor, discuss the crosslinking of chitosan and hyaluronan, also known as hyaluronic acid. Hyaluronan is an acidic linear polysaccharide formed of β-1,3 linked dimeric units, the dimeric units consisting of an 2-acetamido-2-deoxyglucose and D-gluconic acid linked in a β-1,4 configuration. These published applications discuss crosslinking the two types of polysaccharides using a carbodiimide reagent.
Hydrogels comprising chitosan derivatives and polybasic carboxylic acids or oxidized polysaccharides, for use in vascular occlusion, are also disclosed in copending U.S. patent application Ser. No. 11/425,280, filed Jun. 20, 2006 by the same inventors as in the present application.
There is an ongoing need for a hydrogel tissue sealant that is not blood or animal protein derived, that consists of biocompatible materials, is dimensionally stable after emplacement in the patent's body, has good sealant and tissue adhesive properties, is of sufficient strength and elasticity to effectively seal biological tissues, that can be readily prepared and used during surgery, and that forms the tissue seal on a timescale compatible with surgery on living patients.